(1) Field of the Invention
The present invention relates to a light-receiving amplifier circuit used for a pick-up of an optical disc such as a CD-R and a DVD-R, and to an optical pick-up device using the same.
(2) Description of the Related Art
FIG. 1 shows an example of a light-receiving amplifier circuit used for a pick-up of an optical disc such as a CD-R and a DVD-R indicated in the Patent Reference 1 (Japanese Laid-Open Patent Application No. 3123708). In this structure, a cathode of a photodiode 21, which converts an optical signal into an electric signal, is connected to a supply-voltage terminal 22; an anode is connected to an inverting input terminal of an operation amplifier 23; a conversion resistance 24 is connected between the inverting input terminal and an output of the operation amplifier 23; an emitter of a PNP transistor 25 is connected to the inverting input terminal of the operation amplifier 23; a base is connected to the output; and a collector is connected to a GND.
In such structure, when an optical signal is inputted to the photodiode 21, photocurrent, which is proportional to the intensity of that optical signal, is inputted to the operation amplifier 23; the photocurrent is converted into a voltage signal by the conversion resistance 24; and voltage Vo obtained as a product of photocurrent Iph and a resistance value Rfd of the conversion resistance 24 is outputted.
When the optical input is increased and the product of the photocurrent Iph and the resistance value Rfd becomes greater than a base-emitter voltage VBE of the PNP transistor 25, the PNP transistor 25 starts to be active so as to restrict the output voltage based on the VBE. Furthermore, excess current generated at the photodiode 21 because of the further increase in the photocurrent is emitted to the GND so as to prevent the light-receiving amplifier circuit from being saturated.